The Canadian Journal of Chemical Engineering, Volume 81, June-August 2003 1

The present review roughly covers the publications that haveappeared since Prof. K. Schügerl delivered a keynote address atthe 4th GLS International Conference in 1997 (Schügerl, 1997). It

includes publications that deal with gas/liquid systems without solids,but gives an extra weight to applications of airlift bioreactors towastewater treatment and to algal growth. Three extensive reviews havebeen published lately (Chisti, 1988; Merchuk and Gluz, 1999; Petersenand Margaritis, 2001). Most of the publications selected for this revieware not considered. The material is presented under several subtitles‘fluid dynamic characterization’ briefly describes some of the reporteddata on gas holdup, liquid velocity and mass transfer in airlift reactors. Asection on ‘models’ follows. This is a field in which several very relevantpapers have been published. These papers are critically reviewed, tryingto enlighten common characteristics and differences among theapproaches adopted. ‘Wastewater treatment’ is discussed next, becauseof the relevance of the subject and of the close relationship to themodeling section, since some of the most interesting mathematicalmodels were developed in relation to wastewater. A short review of someof the design modifications proposed is then presented, followed by areview of some of the bioprocesses that have been developed choosingairlift reactors. Among these, algal growth has been given special consideration.

Fluid-dynamic CharacterizationThe largest amount of publications related to airlift reactors duringrecent years, report on measurements of fluid dynamic characteristicsand mass transfer rates. Some of them are valuable confirmations orextensions of prior knowledge, and some of them report experimentalresults in modified airlift design, which usually cannot be represented byprior correlations. In any case, the need for collecting and unifying allthose experimental results still exists. We suggested years ago a unifiedformat for airlift data collection and correlation (Har-Noy et al., 1997).

Al-Masri and Abasaeed (1998) did extensive measurements of liquidvelocity and riser holdup in a series of external loop airlift reactors, andproposed empirical correlations that fit their data better than otherproposed correlations. Still there is here a lack of a massive bank of datafor validation of those correlations.

One of the most basic characteristics of gas/liquid dispersions is thebubble size. Couvert et al. (1999) have presented a series of very carefulmeasurements of liquid velocity, riser and downcomer gas holdup andSauter-mean bubble diameter. The last measurements were related tothe gas pressure in the tubular membranes that acted as spargers, and

*Author to whom correspondence may be addressed. E-mail address: jcm@bgumail.bgu.ac.il

Airlift Bioreactors: Review of Recent Advances

By José C. Merchuk*

Department of Chemical Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel

therefore it is possible that the measured resultscannot be extrapolated to coalescing systems withother type of gas spargers. Nevertheless, their two-phase hydrodynamic study is quite complete, and fitsthe model that they present.

Bendjaballah et al. (1999) made a study of liquidvelocity and gas holdup in the riser of an external loopairlift reactor. They used a valve in order to manipulateliquid velocity independently of gas superficialvelocity, and compared a single-orifice gas spargerand a multiple-orifice gas sparger. Their resultsresemble very closely those published by Stein andMerchuk (1981), with the difference that novariations of holdup along the riser were measured

Airlift reactors are popular in the modernbioprocess research and development, over a broadspectrum of processes. Those range from the productionof very expensive biochemicals to wastewatertreatment. In both extremes the reason behind theselection of the airlift reactor is related to fluiddynamic characteristics. During the last few years, alarge number of research papers dealing with thedependence of reactor performance on liquid flowhave been published. In some of them, descriptions ofdevices that have been developed in order to improve,or to gain further control of the fluid patterns in thereactor are reported.

Les réacteurs airlift sont couramment rencontrésdans la recherche et le développement debioprocédés modernes, et ce pour une vaste gammede procédés allant de la production de produitsbiochimiques très chers au traitement des eaux usées.Dans ces procédés, le choix du réacteur airlift est liéaux caractéristiques dynamiques du fluide. Cesdernières années, de nombreux articles de recherchetraitant du lien entre la performance du réacteur etl’écoulement des fluides ont été publiés. Certainsd’entre eux donnent des descriptions de systèmes misau point dans le but d’améliorer ou de mieuxcontrôler les profils d’écoulement dans le réacteur.

Keywords: airlift bioreactors, fluid dynamics, models,wastewater treatment, novel design, photosynthesis.

here. The interesting point in this work is the use of the graphsdisplaying gas slip velocity versus overall fluid velocity in orderto make a quantitative determination of the transition fromhomogeneous gas flow, where the slip velocity decreases withthe total flow, to heterogeneous gas flow — where it increaseswith the total flow. Figure 1 shows the transitions for threedifferent diameters of the downcomer, each giving a differentresistance to liquid circulation. Similar results were found bypartially closing the valve in the downcomer. The same groupattacked the problem of regime identification from a quitedifferent angle (Vial et al., 2001), proposing the use of auto-correlation functions of wall-pressure fluctuations foridentification of flow regimes in airlift reactors. This methodseems to be experimentally simple and very promising for thispurpose.

Van Benthum et al. (1999c) studied the transitions betweengas recirculation regimes in airlift reactors that they define asfollows: Regime 1 (no gas recirculation), Regime 2 (stationarybubble front in the downcomer) and Regime 3 (gas recirculation).It is apparent that Regime 2 has little interest from an industrialpoint of view. The gas that stays practically stationary in thedowncomer will not have much influence on mass transfer,since it will fast become depleted of oxygen, or othercomponents of interest. The authors made a systematic study ofthe effect of suspended solids and liquid throughflow, andmanaged to define criteria for the transition from Regime 2 toRegime 3. The transition depends on the suspended solids andon the throughflow of liquid.

See et al. (1999) made a careful study of the effect of dragand frictional losses on the hydrodynamics of airlift reactors, toelucidate in which measure the use of available methods andcorrelations are applicable to airlift reactors. They recommend acareful evaluation of the frictional losses in all the elements ofthe circuit, including straightening vanes, gas liquid separator,and riser-wall friction. On the other hand they found that theuse of correlations for fully developed single-phase flow in thedowncomer (their external loop reactor has no gas recirculation)does not introduce much error.

Effects of Solids on Mass Transfer RateThe influence of solids on oxygen transfer rate was studied byFreitas and Texeira (1998, 2001). They studied extensively theinfluence of suspended solids on oxygen absorption rate,

changing both solid density and solid loading. The masstransfer rate was calculated assuming that the reactor can beconsidered a perfectly mixed volume. The correctness of thisassumption can be easily proven by the criterion given in thenext section as Equation (3). Taking as an example a gas superficial velocity of 0.2 m/s, representative values of theirmeasured circulation time and volumetric mass transfer coefficientwould be, respectively, 10 s, and 0.01 s–1. Thus in Equation (3)b~10>>1, and the system indeed behaves as a perfectly mixedvessel. This will be the case in any airlift reactor with a relativelyshort draft tube. Their results can be very useful because theycan be considered as obtained in a perfectly mixed system, thuswell defined, and can therefore be used also in the design of tallbioreactors that require separate consideration of each definedfluid dynamic region (structured reactor model).

ModelsAirlift reactors — especially the fluid dynamics of airlift reactors— have a strong appeal and there are many researchers thathave tackled the task of describing the flow of liquid, gas and solids in these bioreactors. In Table 1, some of the modelsthat have been published during the last six years arepresented. Twelve of them model gas/liquid (G/L) systems, and eight attempt the representation of three-phase,gas/liquid/solid (G/L/S) systems. Some overlapping exists, sinceseveral models are presented in a general form that fits bothG/L and G/L/S systems.

The table shows clearly that the liquid velocity and the gasholdup in the riser are seen as the main variables studied andappear in all the proposed models. Indeed, those variables areclosely related, since the difference in gas holdup between riserand downcomer is the only driving force for liquid circulation inthe system. This has been clearly stated by Heijnen et al. (1997),which expressed the pressure difference per unit heightrequired as:

In closed reactors, or as long as the overall solid amount doesnot change, the last term is the product of two differences thathave always the same sign, since if the density of the solid islarger than that of the liquid, its rising velocity will be smallerand the holdup of solids in the riser will be larger than in thedowncomer, and vice versa. The presence of solids, therefore,will always diminish the driving force for circulation, independ-ently of their density.

The hydrodynamic model presented by Heijnen et al. (1997)is one of the most interesting models, and offers a completerepresentation of the fluid dynamics in an airlift reactor from amacroscopic point of view. The model is able to describe bothtwo- and three-phase flow. The ingenuity of the model consistsin avoiding the need of dealing with four different variables (theholdups of gas and liquid in both riser and downcomer) bywriting the difference in gas holdups eGr-eGd as a function of theoverall gas holdup eG, the gas superficial velocity and thecirculation liquid velocity. A similar procedure is followed withrespect to solids holdup. One point that may be criticized in thismodel is the use of the simplification proposed by Chisti (1989),which assumes that the ratio of the gas holdups in downcomerand riser is constant. This is an approximation that seems to be

(1)DPgH Gr Gd Sr Sd S L= -( ) - -( ) -( )e e e e r r

2 The Canadian Journal of Chemical Engineering, Volume 81, June-August 2003

Figure 1. Variation of slip velocity VGL with total gas-liquid velocity fordifferent downcomer diameters. The minimum in each curve indicatestransition from homogeneous to heterogeneous flow. Adapted fromBendjaballah et al. (1999).

close to reality in two-phase systems, though, it contradictssimple liquid mass balance. This has been recognized by Chistihimself (Contreras et al., 1998).

In the case of three-phase flow, the point becomes morecomplicated. A liquid balance at the top results in:

and the range over which a and b can be considered constantsis still an open question.

Seven of the models listed in Table 1 consider gas recircula-tion. Since Heinjen’s model (Heijnen, 1997) circumvents theneed of knowing the gas holdup in riser and downcomer, inspite of handling the regime of gas recirculation, it does notprovide an evaluation of the fraction of gas recirculated. Themodel presented by Freitas et al. (1999) uses a pseudo-homogeneous liquid approach, and assumes that the concen-tration of solids is homogeneous all over the reactor. This isstrictly valid only for solid density that is very close to the densityof the liquid, rS~rL. Indeed, they use alginate beads as solidcarriers, with density close to that of water. Friction coefficientsare calculated from one-phase correlations. The model confirmsthat an increase in solids produces a decrease on holdup. Thegas holdup in the downcomer is calculated as in the model byHeijnen et al. (1997), assuming a linear relationship betweenriser and downcomer holdups, or in other words, constantvalues for a and b in Equation (2). For the external loop theyassume no gas recirculation. They optimize their parameters (4parameters), and get excellent fitting of their data.

Tall airlift reactors constitute a special type because the highliquid velocities that can be reached. Not only can gas be

(2)e e e e ae bGdLr r

Ld dGr

Lr r

Ld dSr Sd Gr

V AV A

V AV A

=È

ÎÍ

˘

˚˙ -

È

ÎÍ

˘

˚˙ -( ) + -( ) = +1 1

entrapped into the downcomer, but it can be directly injectedin this section, with a considerable gain in gas phase residencetime and in energy required for gas injection. One of the mostpromising applications is the disposal of carbon dioxide fromflue gas by injection into deep waters (Kosugi et al., 2001).Sanders et al. (2001) have presented a model for the predictionof liquid velocity in such reactors. A basic analysis based on theZuber and Findlay (1965) two-phase flow approach seems torepresent satisfactorily the trends of pressure drops and liquidvelocities in a high-recirculation airlift reactor.

Garcia Calvo et al. (1999) presented a thermodynamic model(based on the first principle), which is simply an extension of aprevious two-phase model (Garcia Calvo, 1989), to the three-phase airlift reactor (TPAL). They simply replace the liquiddensity in the original model by the liquid-solid pseudo-homogeneous phase density. The model requires knowledge ofthe gas slip-velocity. They show that it is possible to predict thetransition from packed bed to fluidized bed and to circulatingbed, as well as the hysteresis phenomenon that has beenobserved in these flow configurations (Heck and Onken, 1988),and good concurrence with experimentally observed transitionsis reported. These transitions between flow configurationsclassified taking the solid phase as reference, are different fromthe transitions between configurations based on the gasbehavior, as those described by van Benthum et al. (1999c).

Mousseau et al. (1998) present a model for a rectangularsection airlift reactor with suspended solids of lower densitythan water, used for wastewater treatment: the model allowedthe prediction of the changes in ammonia, dissolved oxygenand biomass. They use the penetration model and the isotropicturbulence model for the description of the gas-liquid mass transfer.

In three of the papers in Table 1, the problem of gas/liquidmass transfer is addressed and is given more attention than fluid

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Table 1. Selected models for airlift fluid dynamics and mass transfer

2- 3- Gas Source Phase Phase recirculation VL eGr eGd eSr eSd eG CFD Remarks

Heijnen et al.(1997) x x x x x 3 regimesvan Benthum et al. (1999a) x x x x ALR and

extensionGarcia-Calvo et al. (1999) x x x x xHwang and Lu (1997) x x kLaFreitas et al.(1999) x x x x x x x x es constantMudde and Van der Akker (2001) x x x xCamarasa et al.(2001a) x x xCamarasa et al.(2001b) x x xMousseau et al. (1998) x x x kLa,

ammoniaSáez et al. (1998) x x x x xMárquez et al. (1999) x x x x Axial

profilesCockx et al.(1997) x x x x xCouvert et al.(2001) x x x xTobajas et al.(1999) x x x kLaCamacho-Rubio et al.(2001) x x x x x O2 axial

profilesSanders et al. (2001) x x x x xShechter et al.(2002) x x x x xOrejas (1999) x xSteiff et al.(1997) x x x x x

dynamics: Hwang and Lu (1997), Tobajas et al. (1999) andCamacho Rubio et al. (2001). Hwang and Lu (1997) present ananalysis of “gassing out” experiments, where a different value ofthe mass transfer coefficient kLa is considered in each of thethree distinct regions: riser, downcomer and separator. This is infact an improved version of the original paper by Merchuk et al.(1992), where this structured analysis of an airlift reactor wasfirst presented, structured meaning that zones of different fluiddynamic characteristics are recognized. Those are identifiedwithin the reactor, each of them having potentially a differentgas holdup, liquid dynamics and mass transfer coefficient kLa.Knowledge of the liquid velocity, gas holdups and liquid disper-sion coefficient are required for the use of the Hwang and Lu(1997) model.

The model by Tobajas et al. (1999) refers to a three-phasesystem, since it deals specifically with the biotreatment ofmarine sediment. A simple model is proposed combiningHigbie’s penetration model and Kolmogoroff’s theory ofisotropic turbulence, and is used for prediction of mass transferrates. No parameters are adjusted, and slip velocity, frictioncoefficients, equivalent length, etc. are taken from independentsources. An excellent match of experimental and predictedvalues was found for gas holdup in the riser, liquid velocity andmass transfer rate.

Camacho Rubio et al. (2001) deal with the axial profiles ofoxygen in tall airlift reactors, which are frequently ignored. Theyshow that, as a consequence of the hydrostatic pressurevariation, steady-state axial dissolved oxygen concentrationprofiles exist even if no oxygen consumption exists. These axialinhomogenities increase as the gas flow rate increases. It isobvious, therefore, that the conventional method of kLadetermination, which is based on the assumption of perfectmixing in the reactor, is not valid in a tall bioreactor. Theproblem of inhomogeneity in dissolved oxygen concentrationwas touched also by Dhaouadi et al. (2001), and Korpijarvi et al.(1999), starting from the dynamic response of an airlift reactorto sudden changes in inlet gas concentration. In both cases theyalso made measurements of oxygen concentration and derivedthe mass transfer coefficient using the axial dispersion model. Inall those cases, the main point of interest is the recognition thatthe common assumption of perfect mixing may not hold in thecase of tall reactors.

This had been previously analyzed by Siegel and Merchuk(1990). The criterion proposed there was that the parameter b,ratio of mass transfer characteristic time to liquid circulationtime, should be:

In order to be able to consider the system as perfectly mixed.Tall reactors will have longer circulation times, and the inequalityabove will not be fulfilled. In these cases, the calculation of themass transfer coefficients from experimental data should becarried out using models like those proposed by CamachoRubio et al. (2001) or Hwang and Lu (1997).

Table 1 lists two papers that deal with the application ofcomputational fluid dynamic codes to the description of theflow in airlift reactors. Cockx et al. (1997) presents a simulationbased on one-dimensional two-fluid mass and momentumbalances. Their results produce axial profiles of gas holdup in

(3)bk atL c

= >11

both riser and downcomer, and a flow map of the liquid thatincludes radial profiles of velocity. Mudde and Van Den Akker(2001) carried out two- and three-dimensional simulation of an airlift, trying to minimize the use of ad-hoc closure terms. Surprisingly, they do not find clear advantage in the three-dimensional model, since both approaches coincidewith experimental measurements of gas holdup. The twodimensional model predicts a strong influence of the sparger geometry, and seems to be better at low gas superficialvelocities.

Camarasa et al. (2001a, and b) developed a model forexternal-loop airlift reactors (no gas recirculation) based on acareful momentum balance, using the drift-flux model (Zuberand Findlay, 1965) for gas holdup prediction. The model fitsfairly well their experimental results of mean velocity and gasholdup in the riser, as well as axial variations of the local gasholdup along the riser. They combined this fluid dynamicinformation with a cells-in-series model for mass transfer, whichalso addresses the mixing behavior of the phases.

Couvert et al. (2001) present a simple model for the predictionof liquid velocity and gas holdup in rectangular airlift reactors ofdifferent scales. They do consider gas recirculation, and theirmodel predicts gas holdups in both riser and downcomer,which are close to the measured values.

The fundamental and solid model presented a decade ago byYoung et al. (1991) for an external airlift reactor (1991) wasupgraded (Sáez et al., 1998), adding to the initial model theeffect of gas buoyancy forces in the gas. Based only on thephysical properties of the gas and liquid phases, the reactordimensions and the gas input, the model predicts gas andliquid velocity and gas holdup along the riser for bubbly flow.Total disengagement of the gas at the top is assumed.

The seminal model by Ho et al. (1977) has been an inspirationand a basis for much improved models that represent the airliftreactor as a sequence of perfectly mixed cells (Camarasa et al.2001b; Steiff et al., 1997; Orejas, 1999). Those models areconvenient for handling biological or chemical reactions. Thesimplification of the mathematical treatment may sometimesoutweigh the unrealistic stepwise change of the variables along the reactor that is obtained. Steiff et al. (1997) produceda considerable amount of experimental data that can be usedto calibrate the model parameters. Of special interest, sincethese data are scarce in the literature, are the data of gasrecirculation, measured following the technique proposed bySiegel et al. (1986).

Shechter et al. (2002) presented a three-phase model for thefluid dynamic description of an airlift unit, part of an overallprocess (referred to below). The model is based on amomentum balance over riser and downcomer, and a series ofmacroscopic equations that describe the continuity of liquidand solid as it passes from the riser to the downcomer. Figure 2shows the influence of solid loading on the variables of thesystem: liquid and solid velocity in the riser (the correspondingvalues in the downcomer are not shown to avoid overchargingthe figure), gas and solid holdup in the riser and solids holdupin the downcomer. The model predicts a small increase in bothgas holdup and solids velocity as the amount of solids increasesat constant gas superficial velocity. The solids holdup in boththe riser and the downcomer increases as expected, and theincrease in the downcomer is much sharper than in the riser.The most affected variable is the liquid velocity, ULr, whichdecreases almost to half due to the change in solids loading.

4 The Canadian Journal of Chemical Engineering, Volume 81, June-August 2003

Another interesting simulation in Figure 3 shows the effect ofsolid density on the behavior of the system: while the gasholdup in the riser and the velocity of the solids in the riserremain almost constant when the density of the solid changesby 5% under and over the density of water, the solids holdupin the riser and downcomer are affected in opposite ways: whileeSr increases eSd decreases. The lines cross at a particle densityequal to that of water. The difference between riser anddowncomer solids holdup changes thus from negative topositive as rs increases. The difference in absolute terms issmaller (approximately 0.03 versus 0.074) at higher particledensity, and the predicted increase in liquid velocity, therefore,concurs with the general conclusions from Equation (1). Thissimulation refers to solids with density close to that of theliquid. When the density of the particle is substantially higher,the additional problem of complete fluidization appears. This isespecially critical in systems with short draft tubes, as shown byKojima et al. (1999)

Airlift Reactors in Wastewater TreatmentSince one of the recognized characteristics of airlift reactors isthe potential for scaling up and the relatively low powerconsumption for agitation and oxygenation, it is only naturalthat many processes related to wastewater treatment use thistype of reactor.

Jin et al. (2002) used an airlift reactor in a comprehensivepilot plant system for starch processing wastewater (SPW)reclamation. The starch was utilized by Aspergilius orizae.Simultaneously to a 95% COD, 93% BOD and 98% suspendedsolids removal, an important production of a-amylase (~50EU/ml) was obtained. An interesting point in this paper is thedependence of fungal morphology on ALR fluid dynamics. Inthis type of processes, morphology of the fungal biomass isextremely important. Free mycelial growth (wild growth)increases strongly the viscosity, limiting the oxygen transfer rate

from the gas to the culture. The solution that has been almostuniversally adopted for this problem — which attains to citricand other organic acids, antibiotics, etc. — is to find theconditions under which the biomass takes the form of fungalpellets. The advantage in gas-liquid transfer rate, because of thedecrease in viscosity, usually outweighs the added transferresistance stemming from the intraparticle diffusion of oxygen.But the formation of pellets in optimal size and compactness isa very complex matter. Metz and Kossen (1977) pointed outthe multiplicity of variables and the difficulties of a priori predic-tion of the operation conditions required. Not much has beenadvanced in this matter since. Jin et al. (1999) found empiricallythe optimal gas flow rate in their 4.5 L ALR. It is worthwhile tonote that they seem to have been able to scale up theseconditions to their pilot-plant 160 L ALR.

Lazarova et al. (1997) studied experimentally the fluiddynamics and the performance for wastewater treatment of ansplit-vessel airlift with a rectangular section. They studiedcarefully the influence of suspended solids on gas holdup bothin the riser and the downcomer, as well as the influence of the ratio of riser to downcomer cross sectional areas on liquid velocity. They compare the experimentally measuredvelocities for different reactor heights without proposing anycorrelation. Their measurements of mass transfer rate do notreveal important changes with respect to those obtained inwater once the biofilm is developed. The main aspect stressedby the researchers is the capacity for nitrification observed invarious stages.

Many applications of ALR have been reported in processeswhere the point of interest here is simply that the process,which can take place in a conventional stirred tank, can be runusing an ALR as well, with the consequent savings in energyrequirements, etc. For example, the use of Aspergilius niger fortextile wastewater (biological decoloration) was reported byAssadi and Jahangiri (2001). Campos et al. (2002) used an ALRin a combined (microfiltration and biological) treatment ofoilfield wastewater treatment. They obtained satisfactory resultsin TOC and COD reduction in a continuous process, using an

The Canadian Journal of Chemical Engineering, Volume 81, June-August 2003 5

Figure 2. Influence of solids loading on riser gas holdup (er), riser anddowncomer solids holdups (eSr and eSd), liquid and solid velocity in theriser (ULr and USr), in a rectangular section airlift reactor of a watertreatment system, as predicted by the model by Shechter et al. (2002).The superficial gas velocity was 0.006 (m/s), the area ratio (Ar/Ad)=2.85, the solid density 950 (m/s) and the liquid density 955 (m/s). Theline-identifiers are: � = er (-) ; � = eSr (-) ; � = eSd (-) ; x = ULr (m/s) ; *= USr (m/s).

Figure 3. Influence of particle density on riser gas holdup (er), riser anddowncomer solids holdups (eSr and eSd), liquid and solid velocity in theriser (ULr and USr), in a rectangular section airlift reactor of a watertreatment system, as predicted by the model by Shechter et al. (2002).The superficial gas velocity was 0.006 (m/s), the area ratio (Ar/Ad)=2.85, the solid density 950 (m/s) and the liquid density 955 (m/s). Theline-identifiers are: : � = er (-) ; � = eSr (-) ; � = eSd (-) ; x = ULr (m/s) ; *= USr (m/s).

ALR with suspended polystyrene particles using hydraulicretention times from 12 to 48 h. Both the above-mentionedstudies were carried out in small-scale reactors.

Loh and Liu (2001) used an external loop fluidized bed airliftbioreactor (EIFBAB) for treatment of high strength phenolicwastewater. To control the oxygen transfer they used theincrease in gas holdup that they got by closing a valve in thedowncomer and restricting liquid circulation. The range ofvariation in their device goes in fact from holdup in an airliftwith unrestricted circulation to the holdup in a bubble column,for similar diameters and gas superficial velocity. Obviously, thecase of a completely closed valve implies that the downcomervolume will not contribute to the process.

Bakker et al. (1996) immobilized their biomass inside k-carageenan gel beads, and studied a cascade of two smallscale ALRs to study the oxidation of nitrite to nitrate byNitrobacter agilis. This is an important step in the nitrificationprocess (i.e., the oxidation of ammonia to nitrate via nitrite,usually followed by a denitrification stage with reduction ofnitrate to N2). They found advantages in the use of two bioreactorsin series, and attributed it to the kinetics of the process (non-competitive substrate and product inhibition). Becausethe density of the beads was close to unity, there was noproblem in fluidizing of the beads in spite of the small scale.

While the basic characteristics of an airlift reactor indicate itsfitness for aeration of large volumes of wastewater, the problemof ammonia removal calls for special handling. Two approacheshave been presented lately incorporating the nitrification-denitrification element into a basic airlift arrangement. The firstone is the already mentioned biofilm airlift suspension extension(BASE) reactor (van Benthum 1999a, 1999b), which present thevery compact design that can be seen in Figure 4. The conventionalairlift with its three phases is enclosed into an additional vessel(extension) that becomes the anaerobic volume. Part of theliquid and suspended biofilm coated solids overflows theaerobic airlift core and enter the top of the extension, reentering atthe bottom. The design allows the control of aerobic/anaerobictimes for the biofilm-coated particles suspended in the systemin order to improve the nitrification/denitrification ofwastewater. The flow of liquid and suspended solids in theextension, which is the anaerobic volume, can be controlledmanipulating the overpressure in the headspace of the reactor.A mathematical model was developed and used for the designof a pilot plant. The experimental results of gas and solidholdups concur satisfactorily with the model.

Garrido et al., (1997) showed that in the BASE (Heijnen et al.,1997) they could manipulate the system to obtain higher nitritethan nitrate, in presence of both Nitrosomonas (Æ nitrite) andNitrobacter strains (Æ nitrate). They present a simple modelbased on diffusion in the biomass film, with homogeneousdistribution of microorganisms. At low O2 tensions, nitrificationis improved. The model assumes zero order reaction forNitrosomonas and Blackman kinetics for Nitrobacter.

A different approach to the integration of nitrification/denitrification in a wastewater treatment process is the onepresented by Shechter et al. (2002). They present an approachaiming at the upgrade of existing wastewater treatment plantsrather than de novo design. The experiments presented wereobtained in a 230 m3 aeration basin that was divided in 9sections, as shown in Figure 5. Seven of these were convertedto airlift operation (the fluid dynamic model of which has beencommented above), with two anaerobic sections. In this case,

therefore, the system is a once-through aerobic/anoxic/aerobic/anoxic/aerobic sequence, with options of recirculationfor improved denitrification. The conceptual differencebetween this approach and the BASE system is that the biomassis not recirculated and remains stationary in each of the ninestages, since solids circulate in each airlift stage, but only liquidpasses from one stage to the other. In the BASE approach, allthe biofilm-coated particles transit cyclically both aerobic andanaerobic sections.

Design ModificationsWe have commented already on some interesting airlift designmodifications, as the BASE shown in Figure 4, where anextension is added to the reactor providing a region of fluid andsolids flow that is completely different from the usual airlift flowregions, and still can be controlled, mainly by the headspacepressure in the main reactor. The GLAD (gas lift advanceddissolution), a completely different concept, is a method forsequestration of low purity CO2 emitted by thermal powerplants (Kosugi et al., 2001). The dissolution tube is approximately200 m long, and the drainpipe reaches more than 1000 m indepth. In fact, this is not a gas lift reactor, since this is a once-through system, and recirculation, a basic characteristic ofairlift reactors, does not exist. It is therefore, a gas-lift pump in

6 The Canadian Journal of Chemical Engineering, Volume 81, June-August 2003

Figure 4. Scheme of the BASE (biofilm airlift suspension extension)system. Arrows indicate the flow direction. Liquid, solids and gas flowfrom the downcomer to the riser and vice versa; Only liquid and solidsare present in the extension of the airlift. Adapted from van Benthumet al., (1999a), with permission.

which gas dissolution is especially important. The scheme of thesystem can be seen in Figure 6.

A very interesting system, where the drastic change is not inthe reactor design but in the system itself was presented by Sajcand Vunjak-Novakovic (2000). It consists in an extractivebioconversion taking place in a four-phase external-loop airliftbioreactor. The aim is the extractive bioconversion of anthroquinones by plant cells (Fragnula alnus). The cells areimmobilized in alginate beads suspended in an aqueousmedium. In the riser, gas and a solvent (silicone oil or n-hexadecane) are injected. The gas bubbles provide the energyfor the circulation of the system, and the solvent drops extractthe anthroquinones produced. Both gas and solvent arecompletely separated at the top and only the aqueous mediumand the suspended alginate beads circulate. Figure 7 shows ascheme of the setup, where the inlet and outlet of gas andsolvent are indicated.

A device that changes sensibly the fluid dynamics in thereactor is the helical flow promoter (HFP) (Gluz and Merchuk,1996). The conventional airlift reactor assures a homogeneousand relatively ordered flow, with all the elements of fluidcirculating in a cyclic pattern from the riser to the downcomer.However, in case of large devices, the radial mixing in thedowncomer may not be sufficient. The HFP consists in a seriesof static baffles that modify the flow in all the reactor (Figure 8).This modified flow has several consequences, all of thempotentially beneficial to the yield of the process:a) The helical movement causes secondary flow, which leads to

an enhanced radial mixing, and therefore more homogeneousdistribution of the suspended cells and dissolved substrates.

b) One of the most important characteristics of the HFP is theenhanced capacity for fluidizing solid particles. Thus, it isespecially useful for a process operating with high celldensities. It has been shown that the minimal gas flow rate forcomplete fluidization of solids in an air-lift reactor may be upto four times lower when using HFP (Schotelburg et al., 1999).

c) The mass transfer rate from the liquid to the suspendedsolids may be enhanced up to 50% due to the higher

relative velocity between the particles and the liquid. Thisproperty can be of relevance if the transference of nutrientsor oxygen to a suspended particle becomes rate controlling.

Schlotelburg et al. (1999) made an extensive study of theinfluence of the HFP on reactor fluid dynamics and mass transferrates, including the effect of liquid viscosity. This was a compar-ative study of a bubble column, an airlift reactor and an airliftreactor with HFP. They found that while the fluidization capacityincreased very much because the presence of HFP, the masstransfer rate diminished. This was attributed to an increasedcoalescence due to the corkscrew-like pattern generated in theriser. On the other hand, the reduction of the mass transfer rate due to a viscosity increase, which was very strong in thecase of the a bubble column, was milder in the airlift and muchmilder in the airlift with HFP. This would indicate someadvantage for use of HFP in processes where a large change inviscosity is expected.

Wu and Merchuk (2003) conducted measurements of fluidflow in the downcomer of an internal loop airlift reactor using anovel optical trajectory tracking system especially developedwith this purpose. Analysis of the experimental results showsthat plug flow exists in this region. Shear stress was alsoanalyzed and was found homogeneous. On the other hand themeasurements made on a similar airlift reactor with HFPrevealed the existence of secondary flow in the downcomer.Figure 9 is the flow mapping of the resultant of the axial andradial components of the liquid velocity, uz and ur in the

The Canadian Journal of Chemical Engineering, Volume 81, June-August 2003 7

Figure 5. The Agar System (Shechter et al., 2002). The existingaeration basin was divided into 9 stages, stages 4 and 9 being anoxic,and the other seven rectangular-section airlift reactors. Sludge is fedinto the first stage, and also into the anoxic sections.

Figure 6. The GLAD (gas lift advanced dissolution), a method forsequestration of low purity CO2 emitted by thermal power plants.From Kosugi et al. (2001), with permission.

downcomer of the ALR with HFP. Inspection of the graphsreveals that for a given axial position z, the radial componentchanges in an almost cyclic way with the azimuth q, beingdirected to the exterior and to the axis alternatively. The sametype of fluctuation of ur is seen along z for a fixed q. Thecomponent in the q direction (not shown here) shows a generaltrend to decrease slowly from the top to the bottom of thereactor. It is apparent that the measured velocities aregenerated by a vortex-like movement of the liquid. The experi-mental evidence seems to suggest a stream coiling around itselfas it descends following a helix around the axis. The experi-mental results are thus a clear indication of the existence ofsecondary flow in the downcomer.

Bioprocess ApplicationsWhile airlift reactors have become very popular in researchinstitutes, the stirred tank reactor remains as the undisputedleader in the realm of industrial bioreactors. That is possibly thereason many publications that compare airlift and stirred tankreactors. Kim et al. (1997) compared the production of b-glucosidase by Aspergillius niger in various bioreactors, andreported that the best results correspond to an airlift reactor.

8 The Canadian Journal of Chemical Engineering, Volume 81, June-August 2003

Figure 7. Extractive bioconversion in a four-phase external-loop airlift bioreactor. In the riser, gas and a solvent are injected. Both gas and solventare completely separated at the top and only the aqueous medium and the suspended alginate beads circulate. From Sajk and Vunjak-Novakovic(2000), with permission.

Figure 8. The Helical Flow Promoters: The case of HFP located at thetop of the downcomer. The grey arrows indicate the flow of the liquid.

However, in some processes, such as the bio-oxidation ofminerals, the airlift seems to be the natural choice because ofthe combination of a satisfactory environment for micro-organismgrowth and fluidization of solids (Ruitenberg et al., 2001).

In the case of growth of shear sensitive cells, as animal orplant cells, the airlift reactor has been long ago chosen as oneof the best, and sometimes the only, solutions. Yuan et al.(1999) presented a modified airlift column that includes anexternal resin column through which the liquid is pumped andreturned to the reactor. Alkaloids are secreted by the Cathrantusroseus cells, which are immobilized in a polyurethane foam inthe riser, and are eliminated from the liquid by adsorption onthe resin column. This enhances noticeably the alkaloid produc-tion by the cells.

Su et al. (1996) cultivated plant cells (Anchusa officinalis)suspended in the medium in an airlift reactor with a “calm”sedimentation zone created by a baffle in the downcomer, toovercome the problem of cells that remain suspended at the

top and do not return to the bulk of the liquid. They report tohave doubled the final concentration of the plant cells, and alsothe amount of secreted proteins to the medium.

Visnovsky et al. (2003) reported a comparison of a concentricairlift reactor and a stirred tank bioreactor, for the growth ofUFL-Ag-286 insect cells, and for the production of Anticarsiagemmatalis multiple nucleopolyhedrovirus (AgMNPV). While norelevant differences could be observed in the doubling time andviability of the insect cells, important differences were found inthe kinetics of adsorption of AgMNPV-NOVs (non-occludedvirus) on the insect cells, and in the rate of production ofAgMNPV-OVs (occluded virus). The NOVs were more quickly and efficiently adsorbed on the cells in the airlift reactorthan in the stirred tank reactor. The onset of OVs productionwas earlier in the airlift reactor than in the stirred tank reactor(Figure 10). In addition, the titers of OVs obtained in the airliftreactor were slightly higher than those obtained in the stirredtank reactor for both baculovirus progenies. These resultsindicate clearly the influence of reactor fluid dynamics on theperformance of the process.

Airlift Reactors for Algal CultureSanchez Miron et al. (2000) have focused their study on airliftbioreactors for algal culture. They studied extensively the fluiddynamics (liquid velocity, riser and downcomer holdup andmass transfer coefficient) in a concentric-tubes airlift reactor,and a split-column reactor and compared the results with thosecorresponding to a bubble column of identical dimensions. Theaim of the paper was to relate algal growth to aeration rate, gasholdup and liquid velocity. However, their results ofPhaeodactylum tricornutum growth gave essentially the sameresults for the three reactors, in spite of the differences in thefluid dynamic and mass transfer characteristics between thethree bioreactors that they measured.

This is in contrast to the results presented by Merchuk et al.(2000), which cultivated the red microalga Porphyridium sp. in abubble column, an airlift reactor and an airlift reactor with HFPof similar geometry, and found clear differences among the

The Canadian Journal of Chemical Engineering, Volume 81, June-August 2003 9

Figure 9. Flow mapping of the resultant of the axial and radialcomponents uz and ur in the downcomer of the ALR with HFP withsuperficial gas velocity of 0.015 m/s, and draft tube diameter of 0.12 m.(Wu and Merchuk, 2003).

Figure 10. Comparison of a concentric airlift reactor and a stirred tankbioreactor for the for the production of Anticarsia gemmatalis multiplenucleopolyhedrovirus (AgMNPV) in UFL-Ag-286 insect cells.Thecultures were infected synchronically at a high multiplicity of infectionin a serum-free medium. The time is given in days post infection. FromVisnovsky et al. (2003).

results obtained in each reactor. The essential differencebetween the reports of these two groups may be attributed todifferent kinetics in the photosynthetic growth process of thetwo different algae used.

Among the factors controlling growth, light availability is themost important one. Light flux decreases exponentially withdistance measured from the irradiated side of the photo-bioreactor.The algae near the irradiation source are exposed to highphoton flux density, which enhances growth rate. The cells atthe core of the reactor receive less light as a result of mutualshading and will show a lower growth. On the other hand,excessive light intensity can damage protein D1 in photosystemII (photoinhibition), and decrease growth rate due to reductionin the number of active “photon traps” (Powles, 1984; Krause,1988). An important feature that must be pointed out is thatgrowth rate can be influenced not only by intensity but also bythe history of the illumination that the cells experience (Lee andPirt, 1981). A periodical change in illumination can alsoenhance growth (Marra, 1978). Previous work (Merchuk et al.,1998) based on the fact that ordered mixing can enhance lightavailability and photosynthesis suggested a general approachfor integrating fluid dynamics with the mathematical description ofphotosynthesis (Figure 11). In this approach, fluid dynamics isthe key factor determining the history of the illuminations of thecells.

The kinetics used was based on the three states model ofphotosynthetic factories (PSF) originally proposed by Eilers andPeeters (1988). The PSF is defined as the sum of light trappingsystem, reaction centers and associated apparatus, which areactivated by a given amount of light energy to produce acertain amount of photoproduct. An important assumption isthat the PSF has three states, the resting state (open), called x1,the activated state (closed), called x2, and the inhibited state,called x3. The PSF in resting or open state can be stimulated andtransferred to the activated state by the capture of a photon.The PSFs in activated state may follow one of two possiblepaths; either receiving another photon, which producestemporarily inhibition, or passing the gained energy toacceptors and starting the photosynthesis, returning then to theopen state. Also the inhibited PSF can recover after some time,returning to the open state. The kinetic constants for each ofthese steps were obtained for Porphyridium species in independentexperiments (Wu and Merchuk, 2001).

The fluid flow pattern in an airlift reactor is relatively simpler than that in a bubble column, which has been treatedlately (Wu and Merchuk, 2002). The fluid flows in a definedcirculation pattern through the channels defined by thegeometry of the ALR. In an internal loop ALR, the fluid is drivento pass the downcomer, the riser, and the separator successivelyas shown in Figure 12. For the photosynthesis process, theregion of riser can be regarded as dark zone, and the rest as alight zone with variable illuminance. In the dark zone, the cellkinetics does not depend on the illuminance, and hence theflow pattern in this region has no influence on the light utilization.It was shown elsewhere that perfect mixing can be assumed inthe gas separator (Merchuk and Yunger, 1990, Merchuk et al.,1996). Since usually the residence time in this region is small,the light history can be satisfactorily approximated as one ofconstant illuminance. The downcomer, however, is the majorregion where the cells are illuminated, and detailed flow patternin this region is of interest. Wu and Merchuk (2003) studied thefluid dynamics in this region with an optical trajectories-tracking

system (OTTS) developed by them, and found practically plugflow in the range of interest. This was therefore the reactordynamics adopted in the mathematical model. It should bementioned that both the experiments and the modelingmentioned relate to the range of interest for photosyntheticprocesses, which corresponds to low gas flow rates. In thisrange, homogeneous, bubbly flow is expected in the riser, andnone or minimal gas recirculation in the downcomer.

10 The Canadian Journal of Chemical Engineering, Volume 81, June-August 2003

Figure 11. Schematic description of the integration of fluid dynamicsand photosynthetic growth, from Merchuk et al (1998). Cells in theilluminated region can capture photons and start the photosyntheticchain. They may also become inactivated due to photoinhibition. Afterbeing transported to the dark region, only the dark reactions ofphotosynthesis and recovery from photoinhibition occur. The cells aretransported cyclically from illuminated to dark zones.

Figure 12. Cyclic light history of cells in the Air Lift Reactor, accordingto the model by Wu and Merchuk (2002). Illuminance is taken a nil inthe riser, a mean value is taken for the separator, and in the riser itvaries with the radius of the column.

According to the discussion above, the light history of a cellin the ALR can be represented by the scheme shown in Figure12. The downcomer is divided into several radial regionsaccording to the prevailing illuminance. The algal cells areassumed to be homogeneously distributed in the downcomer,and the fraction of the cells in each light zone can be calculatedaccording to the geometry of the light zones while the gasseparator can be considered as a perfectly mixed volume, andthe riser as a dark zone.

The model allows the simulation of the photosyntheticgrowth in an airlift bioreactor. Figures 13 to 15 show thoseeffects of airlift configuration parameters on the growth. In thissimulation, the illuminance is constant. Figure 13 is the profileof growth working with different column diameters, at HT = 1.0 m.It shows that the increase in biomass concentration (Dx = xf – x0)diminishes as the column diameter increases. This decrease inthe biomass concentration gain is very sharp at low columndiameter (approximately D < 0.2 m) and much slower for largediameters (D > 0.4 m). For example, if the diameter of thecolumn id increased from 0.5 to 1 m, the gain in concentrationwould decrease in 40%. The volume, on the other hand,wouold increase four-fold. This clearly indicates that a properanalysis of the system should include the costs of biomassseparation.

The value of biomass gain (Dx = xf – x0) decreases also as theratio of Ar/Ad increases. At Ar/Ad=1.46, the increase in biomassis zero, i.e., no net positive growth can be obtained at thiscondition. In Figure 14, the effect of the column height ongrowth is presented. Here the column diameter is fixed to be0.2 m, as in the actual experiments. The growth decreases asthe draft tube height increases, due to the increase in theresidence time in the riser. As the column height increases, thecell stays in the dark riser longer, and this leads to a decrease ingrowth. Another effect is that as the height increases, the shearstress increases also. In Figure15, the effect of Ar/Ad on growthis presented. The column diameter and column height are fixedto be 0.2 m and 1.0 m, respectively. It shows that the growth

first increases slowly as Ar/Ad increases, and reaches the highestpoint at value of Ar/Ad = 0.8. When Ar/Ad goes beyond 1.0, thegrowth drops dramatically. This point should be regard as acritical ratio (Rc). When at smaller ratio (Ar/Ad< Rc), the liquidvelocity in the downcomer is smaller than in the riser, and cellsreceive more light. This may cause some photoinhibition. As theratio increases, the photoinhibition is tempered because alonger dark period is introduced. However, a increase in darktime may lead to shortage of photon capture and the negativegrowth effect of the dark period is enhanced. When the darktime is further increased, the overall growth drops and finallyreaches negative values. The profile indicates that when designingan airlift photobioreactor, the ratio of Ar/Ad, must be chosencarefully to avoid losses in biomass growth. An additionalconclusion that can be drawn from the numerical results shownon Figure 15 is that in spite of the apparent monotony in theinfluence of Ar/Ad in Figures 13 and 14, a maximum of theincrease in biomass exists, at Ar/Ad slightly below 1. Thismaximum is not sharp, and is masked there because of theround values chosen for the parametric representation.

The Canadian Journal of Chemical Engineering, Volume 81, June-August 2003 11

Figure 13. Simulation of the effect of column diameter on cell growthin an ALR. The initial cell concentration was x0 = 8 ¥ 106 (cel/mL), theheight of the column 1 (m), the gas superficial velocity was 0.00331(m/s), and the light intensity I0=250 mE·m–2·s–1. Wu and Merchuk(2003).

Figure 14. Simulation of the effect of draft tube height on cell growth.The initial cell concentration was x0 = 8 ¥ 106 (cel/ml), the gas superfi-cial velocity was 0.00331 (m/s), and the diameter of the column 0.2(m) and the light intensity I0 = 250 mE·m–2·s–1. Wu and Merchuk(2003).

Figure 15. Simulation of the effect of the ratio of cross sectional areasAr/Ad on the cell growth. The initial cell concentration was x0 = 8 ¥ 106

(cel/ml), the gas superficial velocity was 0.00331 (m/s), The height ofthe column was 1 (m), the diameter of the column 0.2 (m) and thelight intensity I0 = 250 mE·m–2·s–1. Wu and Merchuk (2003).

The simulations presented here are only qualitative in nature,since they are based on extrapolations. Nevertheless, the trendsshow the potential of this model for simulation and design of alarger scale airlift bioreactors for algal biomass production inclosed systems.

Selection StrategyA point that usually appears at the early stages of the developmentof a system where an ALR is considered is the selection of thetype of bioreactor. Should an external circulation ALR beadopted, or rather an internal circulation ALR?

In the case of bioreactors (except biological leaching ofminerals) fluidization is not a serious problem. The solids haveusually a density that is not far apart of that of water, andfluidization is attained at relatively low liquid velocities.Therefore, the higher liquid velocities that can be attained inexternal loop ALR are not a decisive point. On the contrary,excessive liquid velocity may translate into damages to the cellsdue to shear stress. The main difference remaining is therefore,the presence of gas in the downcomer. In processes where the oxygen concentration in the liquid may be depleted in the downcomer, recirculation of gas will alleviate the problem. Obviously, a wise consideration of the geometricdesign, in order to shorten the residence time in thedowncomer will also contribute.

Our present understanding of the dynamic behaviour of theALR system indicates that rather than maintaining the classicclassification of those reactors into internal and external recirculation devices, attention should be given to the design ofthe top of the reactor where gas separation takes place. Sincemost of times the main element to take into consideration is gasdisengagement, the elements must be balanced will be: a) Thetime that a bubble must spend in the gas separator beforeentering the downcomer (function of the liquid velocity in thegas separator and the length of path of the liquid element), andb) The ascending velocity of the bubble in the liquid. A clearexample would be a split-vessel ALR, that may be seen as an“external recirculation” reactor on one hand, but offers also ashort residence time in the gas separator which provides thebasic characteristics of an “internal recirculation” ALR.

It should be taken into account that this aspect constitutes aserious scale-up problem. As the diameter, or equivalentdiameter of the reactor increases, the path of the liquid elementtraveling from riser to downcomer becomes longer and thechances of bubble disengagement increase too. Ingenuity inthe geometric design has an important role here.

RecapitulationThe distinctive characteristics of airlift reactors are conferred bythe fluid dynamics of the gas/liquid or gas/liquid/solid systemscirculating. These characteristics are usually expressed as gasholdup, liquid and solid velocities and mass transfer rate. It isimportant for the design engineer to recognize whether there isa need to consider those variables separately for each of thedistinct zones of the reactor (structural model). The decision onthis can be made comparing the circulation time and the masstransfer characteristic time (kLa

–1). In any case, only a correctunderstanding of the behaviour and interconnection of riser,separator, downcomer and bottom of the reactor will allow thereliable scale up from the laboratory to pilot or industrial size. Several models that allow the simulation of the fluid dynamicsof airlift reactors have been presented during the last years. It

seems that the mechanisms begin to be clear, and the maindifferences among the models remains in the closure relationships,and in the validity of using in airlift reactors prediction methodsof drag and frictional losses that are generally accepted indeveloped flow.

Several modifications in the design airlift reactors have beenproposed during the last years, and these novel configurationsimprove the performance of the reactors for specific processes,like nitrification/denitrification steps in wastewater treatment,or improve the capacity of fluidization or the radial mixing inthe system.

An interesting application of ALRs is their use as photo-bioreactors, taking advantage of the ordered fluidcirculation to monitor the light/dark cycles of the photosyntheticcells. The influence of the gas input and the geometry of theALR on the performance of the photo-bioreactor can now bepredicted, qualitatively at least.

Many correlations are available for the prediction of gasholdup, liquid velocity, solid circulation and mass transfer rates,and several new ones have been added lately. However, thoseare usually limited to a certain design type, and no generalizedequation with wide range validity exists. A joint analysis of alldata available would probably be of help, but this has not beendone. The designer confronting a scale up or a de novo designmust therefore be extremely careful and analyze the validity ofthe correlations chosen.

NomenclatureAr cross-sectional area of the riser, (m2)Ad cross-sectional area of the downcomer, (m2)b ratio of characteristic time for mass transfer and circulation timeD reactor diameter, (m)g gravitational constant, (m/s2)H draft tube height, (m)I illuminance, or photon flux density, (m.E/m2.s)I(t) illuminance history of a photosynthetic cell, (m.E/m2.s)k yield of photosynthesis production to the transition of x2 Æ x1kLa volumetric mass transfer coefficient, (1/h)r radial distance in the reactor, (m)P pressure drop, (Pa)t time, (s)tc liquid circulation time, (s)T liquid residence time, (s)ur radial component of the liquid velocity, (m/s)uz axial component of the liquid velocity, (m/s)ULr liquid superficial velocity in the riser , (m/s)UGr gas superficial velocity in the riser , (m/s)USr solids superficial velocity in the riser , (m/s)uq circular component of the liquid velocity, (rad/s)VGL gas slip velocity, (m/s)VLr liquid velocity in the riser, (m/s)VLd liquid velocity in the downcomer, (ms–1)x1 fraction of PSF in open statex2 fraction of PSF in close statex3 fraction of PSF in inhibited statex0 initial biomass concentration, (kg/m3)xf final biomass concentration, (kg/m3)Dx increase in biomass concentration (xf- x0), (kg/m3)z axial distance in the reactor, [m]

Greek SymbolseG gas volumetric fractioneGd gas volumetric fraction in the downcomereGr gas volumetric fraction in the risereS volume-averaged solid volumetric fraction

12 The Canadian Journal of Chemical Engineering, Volume 81, August 2003

eSd solid volumetric fraction in the downcomereSr solid volumetric fraction in the riserq circular distance in the reactor, (radians)rL liquid density, (kg/m3)rS solid density, (kg/m3)Dr (rS-rL), (kg/m3)

Subscriptsd downcomerr risers separator

Abbreviations ALR airlift reactorBOD biological oxygen demandCFD computational fluid dynamicsCOD chemical oxygen demandG/L gas/liquidG/L/S gas/liquid/solidHFP helical flow promoterNOV non-occluded virusOV occluded virusPFD photon flux densityPSF photosynthetic factorySTR stirred tank reactorTOC total organic carbon

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14 The Canadian Journal of Chemical Engineering, Volume 81, August 2003

Manuscript received December 4, 2002; revised manuscript receivedJune 4, 2003; accepted for publication June 5, 2003.

All in-text references underlined in blue are linked to publications on ResearchGate, letting you access and read them immediately.All in-text references underlined in blue are linked to publications on ResearchGate, letting you access and read them immediately.